Photovoltaic technologies can be divided into two main groups: flat plates and concentrators. Flat-plate technologies include full area coverage of solar cells between a glass cover and an aluminum (or glass) support. The solar cells can be based on crystalline silicon (from both ingot and ribbon or sheet-growth techniques) or thin films of various semiconductor materials, usually deposited on low-cost substrates, such as glass, plastic, or stainless steel. Deposition techniques typically comprise some type of vapor deposition, electrodeposition, or wet chemical process. In the flat plate technology, solar cells are illuminated directly by sunlight. In concentrator type systems, optical elements are utilized to focus sunlight on to one or more solar cells. For example, a system of lenses and/or reflectors constructed from less expensive materials can be used to focus sunlight on smaller (somewhat more expensive, but highly efficient) solar cells.
Concentrator systems utilize optical elements to focus sunlight before it illuminates a solar cell. A system of lenses or reflectors made from relatively inexpensive materials is used to focus sunlight on smaller, somewhat more expensive, but highly efficient solar cells. Concentration of sunlight by optical means can reduce the required surface area of photovoltaic material while enhancing solar-energy conversion efficiency. One example of a concentrator element is a cylindrical (or cylindrical section) lens that focuses sunlight on to a surface in a linear pattern. By placing a solar cell strip or a linear array of solar cells in the focal plane of such a lens, the focused sunlight can be absorbed and converted directly into electricity by the cell or the array of cells under multi-sun illumination conditions. More electrical energy can be generated from such a concentrator than from a flat plate cell with the same surface area. See, for example, S. M. Sze, Physics of Semiconductor Devices, (John Wiley and Sons, New York, 1981), chapter 14.
Today's solar concentrator systems are generally not compatible with flat-panel modules adapted for residential rooftop installation. Excluding applications as power supplies to satellites and space vehicles, the prior art can essentially be categorized into two types: 1) gigantic individual solar concentrators with the capability of greater than 100 times solar concentration; and 2) panels holding a plurality of concentrators with low or moderate concentration (between about 2 times and 50 times solar concentration) that are fixed to the panel. There are two apparent drawbacks to these prior art approaches when applied to residential roof top installations. First, the concentrators require a tracking mechanism to maintain an optimum angle with incident sunlight ray parallel to the input axis of the concentrator optics. This has been accomplished conventionally by independently rotating individual concentrators in separate frames with separate power sources, or by rotating an entire panel of several concentrators together frozen in the same frame about one or two axes to track the sun's movement. A number of U.S. patents have been granted over the past 30 years directed to this type of approach. Second, most concentrator optics need to be large in size to achieve a high degree of concentration and, therefore, are inappropriate for a flat panel applications having thin solar cell modules. Sun tracking panels are difficult to install on residential rooftops, and current tracking mechanisms which move the entire module frame are bulky and expensive to maintain.
Typically, a solar concentrator or panel of fixed concentrators is not an isolated unit, but rather a part of a larger assemblage deployed in their own frames on a surface of finite dimensions to collect sun power for electricity generation. As a result, one collector will block tracking motions or cast a shadow on the aperture of another collector. To solve this problem, enough frame mounting area (or “real estate”) is provided so that individual solar collectors can be spaced sufficiently far enough apart to minimize shadowing. However, this can create space between collectors where no light can be collected.
Spacing of individual solar cells has a couple of important considerations: 1) what is the appropriate spacing for positioning a plurality of solar concentrators in a given area of finite dimensions, and 2) how should the solar concentrators be deployed, with the assistance of sun tracking, in order to extract as much power as possible from the solar irradiation that falls into that area. When a number of concentrators or concentrator panels capable of tracking the sun's position are grouped together as an array of solar energy collectors, a certain spacing between each individual concentrator is necessary to avoid the shadowing effect caused by the tilting of the concentrators while tracking the sun's movement. This concept is illustrated in FIG. 1. As illustrated in FIG. 1, a plurality of concentrators 10 with aperture dimension 11 of w is separated linearly with an equal distance of d. The spacing between the aperture edges 12 of the adjacent concentrators is g(=d−w). As the aperture normal 13 of concentrators is tilted (e.g., towards the east to face the morning sunshine), the tilt angle ψ 14, which is the angle between the aperture normal and the vertical, can reach a critical point where shadowing does occur. The critical angle ψth is related to the spacing g as follows:
                              ψ          th                =                                            cos                              -                1                                      ⁡                          (                              1                                  1                  +                                      g                    /                    w                                                              )                                .                                    (        1        )            This means that the entire acceptance aperture of every concentrator is fully sun-irradiated as long as ψ<ψth. When the sunlight available to a particular concentrator is partially blocked due to shadowing, the area A(ψ) on the acceptance aperture 15 of a concentrator that is illuminated decreases as the tilt angle increases due to a cosine effect:
                              A          ⁡                      (            ψ            )                          =                  {                                                                      w                                                                      ψ                    ≤                                          ψ                      th                                                                                                                                                              w                      ⁡                                              (                                                  1                          +                                                      g                            /                            w                                                                          )                                                              ⁢                                          cos                      ⁡                                              (                        ψ                        )                                                                                                                                  ψ                    >                                          ψ                      th                                                                                            .                                              (        2        )            This is shown graphically in FIG. 2.
In the real world, solar irradiance to a given area is fixed. The efficiency of solar collection, which can be defined as the ratio of the collectable solar energy to the solar irradiance to the area in question, is the most important factor contributing to overall solar-energy conversion. It is obvious that a large spacing between concentrators, g, wastes useful solar irradiance because a certain part of photon flux from the sunlight (proportional to the spacing dimension) is not collected. The solar-collection efficiency per unit area, or “area efficiency” ηA(ψ), can be expressed as:
                                          η            A                    ⁡                      (            ψ            )                          =                                            A              ⁡                              (                ψ                )                                      d                    =                      {                                                                                1                                          1                      +                                              g                        /                        w                                                                                                                                  ψ                    ≤                                          ψ                      th                                                                                                                                        cos                    ⁡                                          (                      ψ                      )                                                                                                            ψ                    >                                          ψ                      th                                                                                                                              (        3        )            These expressions reveal that the actual solar energy in a given area that can be collected by concentrators is a function of the tilt angle ψ for sun tracking, which is directly correlated to the time. The efficiency also varies with the ratio g/w.
The collectable solar irradiation energy for such a concentrator array at a given tilt angle can be expresses as the product of the solar-collection efficiency ηA(ψ) and the direct solar irradiance I(ψ):W(ψ)=ηA(ψ)I(ψ).  (4)
Note that the direct sun irradiance I(ψ), explicitly expressed here as a generalized case, is a function of ψ. It actually depends on the geographic location, panel orientation, as well as the local date and time. The collectable solar energy by an array of solar concentrators in a given area decreases drastically as the spacing between the adjacent concentrators is increased. Therefore, the answer to the questions above is that when multiple sun tracking concentrators are deployed in a given area of finite dimensions, spacing between adjacent concentrators should be minimized as much as possible, so long as the moving concentrators do not interfere with one another.
While the prior art provides parameters that should be optimized for particular situations, it does not provide satisfactory solutions. In view of the above, a need exists for more efficient concentrators and optical-mechanical tracking systems that can maximize photon capture in a minimum of space. Once light is captured, there remains a need to increase the efficiency of conversion to electricity. It would be desirable to provide maintenance-free concentrator systems designed to be compatible with flat-panel modules and capable of tracking sun movement without relying on rotation of the whole panel. Reduced costs could also be realized through concentrator systems that direct a large window of incident light onto smaller photovoltaic surfaces. What is needed in the art is a third, new type of solution, comprising, and suitable for (e.g.) residential markets. The present invention provides these and other features that will be apparent upon review of the following.